Studies with highly purified preparations of cells from collagenase digests of gastric mucosa suggest that in some species histamine (and histidine decarboxylase) is located in mast cells which lay in close proximity to the parietal cell. In rodents, these substances are present in enthorochromaffin-like (ECL) cells which are embedded in he pit of the gastric gland. A mutant mouse (WB.ReN-Wv) which has few mast cells was found to have near normal histamine levels in brain and stomach and markedly deficient levels (2-8% normal) in other tissues when compared to congeneric normal mice. Studies of the clearance of histamine across the vascular beds of stomach and other organs during i.v. perfusion of histamine in dogs indicated almost complete removal of circulating histamine through the hind limb, partial clearance by visceral organs and a half-life for histamine in the whole animal of 25 sec. Plasma levels of less than 4 x 10 to the minus 7 M histamine appear to be sufficient to induce gastric secretion at 25-50% of the maximal rate.